A mysterious Russian is the prime suspect in the poisoning of former KGB agent Alexander Litvinenko – a tale that has all the ingredients of a first-rate spy thriller.

Litvinenko, 43, told Scotland Yard detectives he arranged a rendezvous Nov. 1 at a central London hotel with an old pal and ex-KGB colleague, Andrei Lugovoy.

But when he arrived, he was surprised to find a second man present – a tall, sharp-featured Russian in his early 40s who introduced himself only as “Vladimir.”

Litvinenko said he became suspicious of the taciturn Vladimir because he revealed nothing about his identity or why he had turned up at a private get-together.

Vladimir repeatedly pressed Litvinenko to join him in a cup of tea but said little else during the brief meeting.

Later that day, Litvinenko became violently ill and suffered major organ failure.

After first thinking he was suffering from a stomach flu, doctors determined he was poisoned – probably with a radioactive substance.

The Times of London reported last night that Vladimir has emerged as the prime suspect in the poisoning.

But Litvinenko was too weak to talk to detectives yesterday and doctors don’t know when he will be well enough to be questioned again.

“I fear he is slipping away before our eyes and doctors don’t know how to save him,” said close friend Alex Goldfarb.

“He is very tired, he is not really communicating. He is aware and alert but the light is irritating him and he is much thinner. He just slips in and out of consciousness and holds on to his wife’s hand.”

Just three weeks ago, Litvinenko was a healthy man with a full head of hair who jogged five miles a day.

Now, his eyes are dark and sunken, his hair has fallen out and his skin and eyes are yellow.

With his immune and nervous systems badly damaged, he is wasting away at London College Hospital after becoming the protagonist in a chilling tale that has all the ingredients of a John Le Carré novel.

These include wealthy Russian émigrés who have turned London into Moscow-on-Thames, ex-KGB agents, the murder of a Russian journalist, the cup-of-tea meeting, a later meeting in a sushi bar, a hit list and, of course, the mysterious poison.

The once-robust Litvinenko’s neck muscles are so weak he can barely lift his head. His skin and eyes are yellow – a clear sign of jaundice.

When he is able to talk, he can only do it in short, painful bursts.

He faces a bone marrow transplant because his body is not producing enough white blood cells to maintain his immune system.

An infection could kill him.

Litvinenko is being guarded by six cops who only allow family and close friends to visit. His wife, Marina, 44, maintains a bedside vigil.

Litvinenko, who worked for the KGB and its successor, the Federal Security Bureau, met Lugovoy, who now provides security for wealthy Russians, in the morning on Nov. 1.

The two became friends when Lugovoy was working as a security officer for the Russian TV station Ort, then owned by billionaire Boris Berezovsky, who is now in exile in Britain and is a fierce critic of Russian President Vladimir Putin.

Later in the day, Litvinenko had a hasty lunch at a Piccadilly sushi bar with Dr. Mario Scaramella, an Italian security expert.

Scaramella, who told reporters Tuesday that he and Litvinenko were involved in an investigation into Russian death squads operating abroad, showed the ex-KGB agent a hit list of Kremlin targets.

Both their names were on it.

Scaramella also passed on details about the killers of Russian investigative reporter Anna Politkovskaya, an outspoken critic of Putin.

The assassins were a group of men from St. Petersburg, Scaramella said.

Litvinenko had publicly blamed Moscow for involvement in Politkovskaya’s death. Later in the day, he was admitted to London College Hospital.

In 2000, Litvinenko, a former FSB colonel who is now a British citizen, published “Blowing Up Russia: Terror from Within,” a book claiming the country’s security services carried out a series of deadly 1999 apartment bombings blamed on Chechen rebels.

In coming to Britain, Litvinenko joined more than 200,000 Russians who have moved there in the past decade.

Akhmed Zakayev, a Chechen rebel commander who lives on the same North London street as Litvinenko, believes Britain no longer offers a haven from his enemies in Russia.

The reason is that in July, the Russian parliament passed a new law allowing the FSB to track down and eliminate enemies of the state.

“The attack on Sacha Litvinenko on Nov. 1 was a direct result” of this law, Zakayev told The Times of London.

The Russian Foreign Intelligence service, the SVR, issued a strong denial yesterday that it wasn’t involved in the attempt to kill Litvinenko.